Spring 注解面面通之 @MatrixVariable引數系結原始碼決議-有解無憂

??Spring MVC中使用HandlerMethodArgumentResolver策略介面來定義處理器方法引數決議器，@MatrixVariable使用的是MatrixVariableMapMethodArgumentResolver和MatrixVariableMethodArgumentResolver，接下來一起來深入了解一下其原始碼實作，

在這里插入圖片描述

??類決議

??HandlerMethodArgumentResolver和AbstractNamedValueMethodArgumentResolver是決議策略的上層定義和抽象，關于這兩個類可以參照《Spring 注解面面通之 @CookieValue引數系結原始碼決議》中的決議，

??MatrixVariableMapMethodArgumentResolver和MatrixVariableMethodArgumentResolver則是用來針對不用型別的方法引數的決議，

??1) MatrixVariableMapMethodArgumentResolver實作了HandlerMethodArgumentResolver的supportsParameter(...)和resolveArgument(...)方法，

??MatrixVariableMapMethodArgumentResolver在某些條件成立的情況下才會使用此類進行決議：

??① 方法引數由@MatrixVariable注解注釋，

??② 方法引數型別必須是Map型別，

??③ 注釋方法引數的@MatrixVariable的name屬性不能有值，

??resolveArgument(...)決議引數從NativeWebRequest.getAttribute( HandlerMapping.MATRIX_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST)獲得，

package org.springframework.web.servlet.mvc.method.annotation;

import java.util.Collections;
import java.util.List;
import java.util.Map;

import org.springframework.core.MethodParameter;
import org.springframework.core.ResolvableType;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.CollectionUtils;
import org.springframework.util.LinkedMultiValueMap;
import org.springframework.util.MultiValueMap;
import org.springframework.util.StringUtils;
import org.springframework.web.bind.annotation.MatrixVariable;
import org.springframework.web.bind.annotation.ValueConstants;
import org.springframework.web.bind.support.WebDataBinderFactory;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.context.request.RequestAttributes;
import org.springframework.web.method.support.HandlerMethodArgumentResolver;
import org.springframework.web.method.support.ModelAndViewContainer;
import org.springframework.web.servlet.HandlerMapping;

/**
 * 決議帶@MatrixVariable注解的Map型別的引數，其中注解未指定名稱. 
 * 換句話說，這個決議器的目的是提供對多個矩陣變數的訪問，這些變數可以是全部的，也可以是與特定路徑變數相關聯的.
 *
 * 指定名稱時，型別為Map的引數被視為具有映射值的單個屬性，并由MatrixVariableMethodArgumentResolver決議.
 */
public class MatrixVariableMapMethodArgumentResolver implements HandlerMethodArgumentResolver {

	/**
	 * 方法引數檢查.
	 * 方法引數需包含@MatrixVariable注解、為Map型別引數，且@MatrixVariable的name屬性不可有值.
	 */
	@Override
	public boolean supportsParameter(MethodParameter parameter) {
		MatrixVariable matrixVariable = parameter.getParameterAnnotation(MatrixVariable.class);
		return (matrixVariable != null && Map.class.isAssignableFrom(parameter.getParameterType()) &&
				!StringUtils.hasText(matrixVariable.name()));
	}

	/**
	 * 決議方法引數值.
	 */
	@Override
	@Nullable
	public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
			NativeWebRequest request, @Nullable WebDataBinderFactory binderFactory) throws Exception {
		// 獲取NativeWebRequest中存盤的HandlerMapping.MATRIX_VARIABLES_ATTRIBUTE的值.
		@SuppressWarnings("unchecked")
		Map<String, MultiValueMap<String, String>> matrixVariables =
				(Map<String, MultiValueMap<String, String>>) request.getAttribute(
						HandlerMapping.MATRIX_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);
		// 矩陣變數為空.
		if (CollectionUtils.isEmpty(matrixVariables)) {
			return Collections.emptyMap();
		}
		
		MultiValueMap<String, String> map = new LinkedMultiValueMap<>();
		MatrixVariable ann = parameter.getParameterAnnotation(MatrixVariable.class);
		Assert.state(ann != null, "No MatrixVariable annotation");
		// 獲取@MatrixVariable指定的矩陣變數的路徑值.
		String pathVariable = ann.pathVar();
		// @MatrixVariable指定的矩陣變數的路徑值不為空.
		if (!pathVariable.equals(ValueConstants.DEFAULT_NONE)) {
			// 根據路徑獲取矩陣變數.
			MultiValueMap<String, String> mapForPathVariable = matrixVariables.get(pathVariable);
			if (mapForPathVariable == null) {
				return Collections.emptyMap();
			}
			map.putAll(mapForPathVariable);
		}
		// @MatrixVariable指定的矩陣變數的路徑值為空.
		else {
			// 設定全部矩陣變數.
			for (MultiValueMap<String, String> vars : matrixVariables.values()) {
				vars.forEach((name, values) -> {
					for (String value : values) {
						map.add(name, value);
					}
				});
			}
		}
		// MultiValueMap轉換為Map.
		return (isSingleValueMap(parameter) ? map.toSingleValueMap() : map);
	}
	
	/**
	 * 方法引數型別是否是MultiValueMap型別.
	 * @param parameter 方法引數.
	 * @return 是否是MultiValueMap型別.
	 */
	private boolean isSingleValueMap(MethodParameter parameter) {
		if (!MultiValueMap.class.isAssignableFrom(parameter.getParameterType())) {
			ResolvableType[] genericTypes = ResolvableType.forMethodParameter(parameter).getGenerics();
			if (genericTypes.length == 2) {
				Class<?> declaredClass = genericTypes[1].getRawClass();
				return (declaredClass == null || !List.class.isAssignableFrom(declaredClass));
			}
		}
		return false;
	}

}

??2) MatrixVariableMethodArgumentResolver繼承自抽象AbstractNamedValueMethodArgumentResolver（可以參照《Spring 注解面面通之 @CookieValue引數系結原始碼決議》），

??MatrixVariableMethodArgumentResolver相對MatrixVariableMapMethodArgumentResolver來說，實作基本相同，其在某些條件成立的情況下才會使用此類進行決議：

??① 方法引數由@MatrixVariable注解注釋，

??② 當方法引數型別是Map時，要求其必須由@MatrixVariable注解注釋，同時@MatrixVariable的name屬性必須有值，

??resolveArgument(...)決議引數從NativeWebRequest.getAttribute( HandlerMapping.MATRIX_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST)獲得，

package org.springframework.web.servlet.mvc.method.annotation;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import org.springframework.core.MethodParameter;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.CollectionUtils;
import org.springframework.util.MultiValueMap;
import org.springframework.util.StringUtils;
import org.springframework.web.bind.ServletRequestBindingException;
import org.springframework.web.bind.annotation.MatrixVariable;
import org.springframework.web.bind.annotation.ValueConstants;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.context.request.RequestAttributes;
import org.springframework.web.method.annotation.AbstractNamedValueMethodArgumentResolver;
import org.springframework.web.servlet.HandlerMapping;

/**
 * 決議用@MatrixVariable注釋的引數.
 *
 * 如果方法引數是Map型別，它將由MatrixVariableMapMethodArgumentResolver決議，除非注釋指定了一個名稱，
 * 	在這種情況下，它被視為Map型別的單個屬性（而不是在映射中收集的多個屬性）.
 */
public class MatrixVariableMethodArgumentResolver extends AbstractNamedValueMethodArgumentResolver {
	
	/**
	 * Constructor.
	 */
	public MatrixVariableMethodArgumentResolver() {
		super(null);
	}

	/**
	 * 方法引數檢查.
	 * 方法引數需包含@MatrixVariable注解.
	 * 若方法引數為Map型別引數，且@MatrixVariable的name屬性必須有值.
	 */
	@Override
	public boolean supportsParameter(MethodParameter parameter) {
		if (!parameter.hasParameterAnnotation(MatrixVariable.class)) {
			return false;
		}
		if (Map.class.isAssignableFrom(parameter.nestedIfOptional().getNestedParameterType())) {
			MatrixVariable matrixVariable = parameter.getParameterAnnotation(MatrixVariable.class);
			return (matrixVariable != null && StringUtils.hasText(matrixVariable.name()));
		}
		return true;
	}

	/**
	 * 創建NamedValueInfo.
	 */
	@Override
	protected NamedValueInfo createNamedValueInfo(MethodParameter parameter) {
		MatrixVariable ann = parameter.getParameterAnnotation(MatrixVariable.class);
		Assert.state(ann != null, "No MatrixVariable annotation");
		return new MatrixVariableNamedValueInfo(ann);
	}

	/**
	 * 決議方法引數值.
	 */
	@Override
	@SuppressWarnings("unchecked")
	@Nullable
	protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest request) throws Exception {
		// 獲取NativeWebRequest中存盤的HandlerMapping.MATRIX_VARIABLES_ATTRIBUTE的值.
		Map<String, MultiValueMap<String, String>> pathParameters = (Map<String, MultiValueMap<String, String>>)
				request.getAttribute(HandlerMapping.MATRIX_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);
		// 矩陣變數為空.
		if (CollectionUtils.isEmpty(pathParameters)) {
			return null;
		}

		MatrixVariable ann = parameter.getParameterAnnotation(MatrixVariable.class);
		Assert.state(ann != null, "No MatrixVariable annotation");
		// 獲取@MatrixVariable指定的矩陣變數的路徑值.
		String pathVar = ann.pathVar();
		List<String> paramValues = null;
		// @MatrixVariable指定的矩陣變數的路徑值不為空.
		if (!pathVar.equals(ValueConstants.DEFAULT_NONE)) {
			if (pathParameters.containsKey(pathVar)) {
				paramValues = pathParameters.get(pathVar).get(name);
			}
		}
		// @MatrixVariable指定的矩陣變數的路徑值為空.
		else {
			boolean found = false;
			paramValues = new ArrayList<>();
			// 設定全部矩陣變數.
			for (MultiValueMap<String, String> params : pathParameters.values()) {
				if (params.containsKey(name)) {
					if (found) {
						String paramType = parameter.getNestedParameterType().getName();
						throw new ServletRequestBindingException(
								"Found more than one match for URI path parameter '" + name +
								"' for parameter type [" + paramType + "]. Use 'pathVar' attribute to disambiguate.");
					}
					paramValues.addAll(params.get(name));
					found = true;
				}
			}
		}
		
		if (CollectionUtils.isEmpty(paramValues)) {
			return null;
		}
		else if (paramValues.size() == 1) {
			return paramValues.get(0);
		}
		else {
			return paramValues;
		}
	}

	/**
	 * 處理引數值缺失例外.
	 */
	@Override
	protected void handleMissingValue(String name, MethodParameter parameter) throws ServletRequestBindingException {
		throw new ServletRequestBindingException("Missing matrix variable '" + name +
				"' for method parameter of type " + parameter.getNestedParameterType().getSimpleName());
	}


	/**
	 * MatrixVariableNamedValueInfo.
	 */
	private static class MatrixVariableNamedValueInfo extends NamedValueInfo {

		private MatrixVariableNamedValueInfo(MatrixVariable annotation) {
			super(annotation.name(), annotation.required(), annotation.defaultValue());
		}
	}

}

??3) NativeWebRequest的
HandlerMapping.MATRIX_VARIABLES_ATTRIBUTE屬性

??分析過MatrixVariableMapMethodArgumentResolver和MatrixVariableMethodArgumentResolver原始碼后，發現矩陣變數都是從NativeWebRequest屬性中獲得，那么是在哪里進行的矩陣模板變數決議呢？

??① 涉及矩陣決議的呼叫程序，

DispatcherServlet.doDispatch(...) --呼叫--> 
DispatcherServlet.getHandler(...) --呼叫--> 
AbstractHandlerMapping.getHandler(...) --呼叫--> 
AbstractHandlerMethodMapping.getHandlerInternal(...) --呼叫--> 
AbstractHandlerMethodMapping.lookupHandlerMethod(...) --呼叫--> 
RequestMappingInfoHandlerMapping.handleMatch(...) --呼叫-->
RequestMappingInfoHandlerMapping.extractMatrixVariables(...) --呼叫-->
根據已決議的URI模板變數，來決議矩陣變數

??② 查看RequestMappingInfoHandlerMapping.handleMatch(...)方法，可以發現，在這里對矩陣變數進行了決議，并設定到NativeWebRequest屬性中，

/**
 * 暴露URI模板變數、矩陣變數和可生成的媒體型別.
 */
@Override
protected void handleMatch(RequestMappingInfo info, String lookupPath, HttpServletRequest request) {
    super.handleMatch(info, lookupPath, request);

    String bestPattern;
    Map<String, String> uriVariables;
    Map<String, String> decodedUriVariables;

    Set<String> patterns = info.getPatternsCondition().getPatterns();
    // 若不存在匹配模式，則均設定為空.
    if (patterns.isEmpty()) {
        bestPattern = lookupPath;
        uriVariables = Collections.emptyMap();
        decodedUriVariables = Collections.emptyMap();
    }
    else {
        bestPattern = patterns.iterator().next();
        // 決議URI模板變數.
        uriVariables = getPathMatcher().extractUriTemplateVariables(bestPattern, lookupPath);
        // 對URI模板變數進行解碼.
        decodedUriVariables = getUrlPathHelper().decodePathVariables(request, uriVariables);
    }
    // 設定匹配模式到HttpServletRequest的BEST_MATCHING_PATTERN_ATTRIBUTE.
    request.setAttribute(BEST_MATCHING_PATTERN_ATTRIBUTE, bestPattern);
    // 設定URI模板變數到HttpServletRequest的HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE.
    request.setAttribute(HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE, decodedUriVariables);
    // 處理矩陣變數.
    if (isMatrixVariableContentAvailable()) {
        // 提取矩陣變數.
        Map<String, MultiValueMap<String, String>> matrixVars = extractMatrixVariables(request, uriVariables);
        // 設定矩陣變數到HttpServletRequest的HandlerMapping.MATRIX_VARIABLES_ATTRIBUTE.
        request.setAttribute(HandlerMapping.MATRIX_VARIABLES_ATTRIBUTE, matrixVars);
    }
    // 處理可生成的媒體型別.
    if (!info.getProducesCondition().getProducibleMediaTypes().isEmpty()) {
        // 獲取可生成的媒體型別.
        Set<MediaType> mediaTypes = info.getProducesCondition().getProducibleMediaTypes();
        // 設定可生成媒體型別到HttpServletRequest的PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE.
        request.setAttribute(PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE, mediaTypes);
    }
}

??③ RequestMappingInfoHandlerMapping.extractMatrixVariables(...)負責矩陣變數的具體決議，

/**
 * 提取矩陣變數.
 * @param request 當前請求實體.
 * @param uriVariables URI模板變數.
 * @return 矩陣變數.
 */
private Map<String, MultiValueMap<String, String>> extractMatrixVariables(
    HttpServletRequest request, Map<String, String> uriVariables) {

    Map<String, MultiValueMap<String, String>> result = new LinkedHashMap<>();
    // 遍歷矩陣變數.
    uriVariables.forEach((uriVarKey, uriVarValue) -> {
        // URI模板變數值中存在=.
        int equalsIndex = uriVarValue.indexOf('=');
        if (equalsIndex == -1) {
            return;
        }

        String matrixVariables;
        // URI模板變數值中存在;，若不存在;或;在首位，或=在;前，URI模板變數的值即是矩陣變數.
        int semicolonIndex = uriVarValue.indexOf(';');
        if ((semicolonIndex == -1) || (semicolonIndex == 0) || (equalsIndex < semicolonIndex)) {
            matrixVariables = uriVarValue;
        }
        else {
            matrixVariables = uriVarValue.substring(semicolonIndex + 1);
            uriVariables.put(uriVarKey, uriVarValue.substring(0, semicolonIndex));
        }
        // 決議矩陣變數到Map.
        MultiValueMap<String, String> vars = WebUtils.parseMatrixVariables(matrixVariables);
        result.put(uriVarKey, getUrlPathHelper().decodeMatrixVariables(request, vars));
    });
    return result;
}

??總結

??以上便是@MatrixVariable對方法引數決議的全程序，對于理解@MatrixVariable原理來說還是十分有用處的，更有利于在實際開發中解決那些疑難雜癥，

??若文中存在錯誤和不足，歡迎指正

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標籤：java

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Spring 注解面面通 之 @MatrixVariable引數系結原始碼決議

Spring 注解面面通之 @MatrixVariable引數系結原始碼決議